This invention relates to a ferromagnetic chromium dioxide product modified by tungsten alone or further with copper and/or zinc to obtain a high coercive force, and to a method for producing the same.
More particularly, this invention relates to ferromagnetic chromium dioxide product suitable for use mainly in magnetic recording media such as magnetic tape, magnetic disc and the like.
As is generally known, a powdery magnetic material having hitherto been exclusively used for magnetic recording media is ferromagnetic iron oxide (.gamma.-Fe.sub.2 O.sub.3).
Quite recently, however, ferromagnetic chromium oxide has become attractive for use as magnetic recording media. As compared with conventionally known .gamma.-Fe.sub.2 O.sub.3, the ferromagnetic chromium oxide comprising chromium dioxide has superior characteristics, particularly superior magnetic characteristics, such as a higher value of saturation magnetization, lower electric resistance, smaller demagnetization under pressure, and less divergence between the direction of the axis of crystal anisotropy and that of the axis of shape anistropy. Nevertheless, chromium dioxide exhibits the greatest disadvantage in a small coercive force, which retarded its coming into practical use. In order to be used in magnetic recording media, chromium dioxide must be improved in its coercive force to 200 Oe. or higher, while, on the other hand, the most important problem has been how the improvement should be done. It was not until recent years that methods for preparing chromium dioxide having a high coercive force began to be made public. It has been known that the following patents, for example, were issued.
Japanese Pat. Publication No. 8839/61 discloses a method which comprises incorporating ruthenium dioxide, an antimony compound, or a tin compound into chromium trioxide, and thermally decomposing the resulting mixture under a high pressure.
U.S. Pat. No. 3,117,093 discloses a method which comprises thermally treating under a high pressure a chromium oxide, in which the chromium atom has a valence greater than 4 and less than 6, and also teaches that a high coercive force can be obtained by incorporating additives such as antimony oxide.
As is apparent from the description of those patents, it has become known that the addition of a certain effective additive during synthesis of said chromium dioxide is an efficient means of obtaining a ferromagnetic chromium dioxide product having a high coercive force.